1. Field of the Invention
This invention relates to a display brightness modulating circuit for the display device of a photographic camera.
2. Description of the Prior Art
There is known a display device of the kind which performs A-D conversion on a display information signal supplied to an A-D converter. This device makes a digital display of the display information signal by supplying the A-D converted value thus obtained to a display circuit. Where a seven segment display arrangement formed with LED's or a plurality of light emitting elements such as LED's are employed in this type of conventional display device and where a display value scale mark is shown with the LED lit up at applicable points on a scale in a manner called a dot display, the display is sometimes affected by the brightness of ambient light and becomes barely observable if the display brightness is invariable. In view of this inconvenience, some display devices have been contrived that have adjustable light emission brightness of the display means based on according to the brightness of ambient light. Display devices of this type are shown, for example, in Japanese Laid-Open Patent Applications Nos. SHO 52-102719, SHO 50-34532 and SHO 49-64426; Japanese Utility Model Publication No. SHO 43-27370; and Japanese Patent Publication No. SHO 55-9690. When brightness modulation is carried out by these devices, a display is made on the basis of A-D converted brightness information. The display value itself represents a brightness value in most cases and brightness modulation can be accomplished by using the value of this A-D converted information.
However, when the display value is either not relative to brightness or a spot photometric value though it relates to brightness, the display light emission brightness cannot be adjusted to an apposite level by controlling the display brightness in the above manner on the basis of the A-D converted value indicates of a display value. To solve this problem, a photometric circuit obtains information on display brightness modulation required and adjusts the brightness of the light emitting element on the basis of a photometric output of this circuit in a manner, for example, disclosed in Japanese Utility Model Publication No. SHO 48-553.
However, in cases where the above display is to be made in a digital manner, it is preferable to employ a brightness modulating method in which the duty of display driving pulses is adjusted in a manner, for example, disclosed in the above Japanese Laid-Open Patent Application No. SHO 52-102719. This display brightness modulating method, however, necessitates A-D converting the output of the above photometric circuit and determining the duty of the display driving pulses on the basis of the A-D converted value thus obtained, as disclosed in the Japanese Laid-Open Patent Application No. SHO 52-102719.
Meanwhile, since the information to be displayed is A-D converted by an A-D converter as mentioned in the foregoing, the above method of A-D converting the brightness modulation information necessitates the use of two A-D converters.
The present invention is directed to the solution of the problem presented by the prior art device mentioned above. It is therefore a principal object of the invention to provide a simplified display device for a camera wherein the input of an A-D converter which is receiving the above display information is switched over from display information to a photometric output produced for display brightness modulation every time the display information is A-D converted a predetermined number of times, so that digital values representative of the display information and a digital value of the display brightness modulation information can both be obtained from a single A-D converter.
It is another object of the invention to provide a camera wherein, even when brightness information is locked by an EE (electric eye) locking operation during the above brightness modulation, the photometric output for display brightness modulation is not locked; and display brightness can be modulated even after the EE lock to ensure that the camera display is easily observable even in the event of an EE locking operation.
It is a further object of the invention to provide a camera wherein, when the input of the A-D converter is switched from one signal to another in the manner described in the foregoing, an A-D converting operation is not immediately performed on the input signal received by switching but is performed after the lapse of a predetermined period of time so that an accurate digital value can always be obtained for the signal received by switching.
These and further objects and features of the invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.